The present invention relates to a surface modification process of a quartz glass crucible and to a surface modified quartz glass crucible which is used in the process of pulling up silicon single crystal used for a semiconductor etc. from molten silicon.
As one of the causes that a dislocation is formed in silicon single crystal in the pulling method, it has been known that cristobalite formed on the inside surface of the quartz glass crucible is released into the molten silicon. As a countermeasure to this, a process is known in which an alkaline earth metal is coated on an inside surface of the crucible to act as a crystallization promoter. The metal forms a cristobalite layer on the inside surface of the crucible at an early stage of pulling up (e.g. U.S. Pat. No. 5,976,247, Japanese Patent No. 3,100,836). In these inventions, a barium hydroxide solution is coated on the surface of the quartz glass crucible and the coated barium hydroxide reacts with carbon dioxide in air to form barium carbonate on the surface of the crucible. The barium carbonate is weakly adhered on the surface of the crucible by drying and is used as a crystallization promoter.
However, quartz glass crucibles surface-treated by conventional methods have the following problems and improvements are desired. That is, (A) since the barium carbonate powder on the quartz glass crucible is not fixed with any binder, the adhesion strength of the powder is very weak so that the powder is easily abraded and falls off when contacted by persons and instruments. Such abrasion occurs in the production process of the crucibles, such as product inspection, conveyance, and insertion to a carrying case, etc. It also occurs in the user""s process, where the quartz glass crucible is set on a carbon susceptor in the crystal puller. As a result, its adhesion state becomes non-uniform and spot-like. Furthermore, there is also a possibility that worker health is negatively affected because barium carbonate powder is scattered when the carrying case is opened. (B) Since the adhesion strength of powder is very weak, nucleation efficiency as a crystallization promoter is low, and so the amount of barium carbonate required becomes excessive. (C) If the crucible is washed, the barium carbonate powder adhered on the surface of the crucible is washed away. It is then impossible to wash the crucible after the adhesion of the barium carbonate powder, even if some contaminants adhere on the surface of the crucible.
The present invention solves the problems of the conventional quartz glass crucible and provides a surface modified quartz glass crucible having a transparent coated layer which is not abraded upon contact with handling instruments or persons, has sufficient durability, and also provides its surface modification process.
That is, the present invention relates to the following surface modification process of the quartz glass crucible.
[1] A surface modification process of a quartz glass crucible, the process comprising, coating a mixed solution, (hereinafter referred to as the silica sol liquid), containing a metal salt and a partial hydrolyzate of alkoxysilane oligomer on the surface of the crucible, baking said coated solution to form a transparent coated layer containing a crystallization promoter derived from the metal salt in the silica matrix.
The surface modification process of the present invention includes the following processes.
[2] The surface modification process of a quartz glass crucible, wherein the metal salt is a metal organic acid salt or a metal carbonate of one or more kinds of magnesium, calcium, strontium, or barium.
[3] The surface modification process of a quartz glass crucible, the process comprising, coating the silica sol liquid on the whole or a part of the inside and/or outside of the surface of the quartz glass crucible, and baking said coated silica sol liquid.
[4] The surface modification process of the quartz glass crucible, wherein the silica sol liquid has 0.01 to 15 weight % of metal content, which is calculated as an oxide, and 0.5 to 30 weight % of silicon concentration, which is calculated as SiO2, is used.
[5] The surface modification process of the quartz glass crucible, the process also comprising, drying the silica sol liquid, and baking said dried liquid at 350 to 1200xc2x0 C. for 10 to 120 minutes.
In addition, the present invention also relates to the following quartz glass crucible.
[6] A quartz glass crucible, having a transparent coated layer, in which the crystallization promoter is dispersed in the silica matrix, on the whole or a part of the inside and/or outside surface of the crucible.
The quartz glass crucible of the present invention includes the following crucibles.
[7] The quartz glass crucible, wherein the crucible is made by, coating the silica sol liquid containing the metal salt and the partial hydrolyzate of alkoxysilane oligomer on the surface of the crucible, and baking the coated silica sol liquid to form a transparent coated layer containing the crystallization promoter derived from the metal salt in the silica matrix.
[8] The quartz glass crucible, wherein the crucible is made by coating the silica sol liquid containing a metal organic acid salt or a metal carbonate of one or more kinds of magnesium, calcium, strontium, or barium, on the surface of the crucible, and baking coated silica sol liquid to form the transparent coated layer containing the metal oxide or the metal carbonate as the crystallization promoter.
[9] The quartz glass crucible used as the crucible for silicon single crystal production.
The surface modified quartz glass crucible of the present invention has a hard and transparent coated layer, in which the metal oxide or the metal carbonate is dispersed in a silica matrix, on the whole or a part of the inside and/or outside surface of the crucible. In addition, the metal oxide or metal carbonate acts as a crystallization promoter to the surface glass layer of the crucible at the high temperature of pulling up the silicon single crystal. Therefore, when the crucible is used for the production of silicon single crystal, a uniform cristobalite layer is formed on the inside surface of the crucible at an early stage of pulling up, and as a result a high dislocation free ratio of the pulled crystal can be obtained. Moreover, the strength of the crucible under a high temperature is increased by the uniform cristobalite layer formed on the inside or outside surface of the crucible.
Moreover, since the coated layer is baked on the surface of the crucible, it is very stable and has high durability. Therefore, there is no abrasion when contacted with instruments or persons and no problem that the adhesion state of the metal oxide in the coated layer becomes to non-uniform. In addition, even if the coating layer is comparatively thin, the surface of the crucible is crystallized uniformly during pulling up the single crystal and the dislocation free ratio of silicon can be increased. On the other hand, regarding the conventional quartz glass crucible having the adhered barium carbonate powder on its surface, since the adhesion strength of the barium carbonate powder is very weak, it can be washed away easily by acid washing. Therefore, it is impossible to wash the crucible even if some contaminants are adhered on the surface of the crucible. While regarding the surface modified quartz glass crucible of the present invention, since the coated layer is baked on the surface of the crucible, it is not washed away by acid washing. Contaminants on the surface of the crucible can be removed easily by acid washing.
Hereafter, the present invention is explained concretely according to the preferred embodiment.
The surface modification process for producing the quartz glass crucible of the present invention is a formation process for a transparent coated layer on a surface of a crucible, where a crystallization promoter is dispersed in a silica matrix. Moreover, the quartz glass crucible of the present invention has a transparent coated layer on the whole or a part of the inside and/or outside surface of the crucible. The crystallization promoter is a metal compound except for silica, which is for example a metal oxide or metal carbonate. Concretely, the metal is for example, magnesium, calcium, strontium, or barium etc.
The transparent coated layer, where the crystallization promoter is dispersed in the silica matrix, can be formed by coating the mixed solution (the silica sol liquid) containing the metal salt and the partial hydrolyzate of alkoxysilane oligomer on the surface of the crucible, and by baking this coated silica sol liquid.
The silica sol liquid forming the transparent coated layer is a mixed solution which contains both a metal salt and a partial hydrolyzate of an alkoxysilane oligomer as components. The metal salt is for example a salt of magnesium, calcium, strontium, or barium, which is a useful metal as a crystallization promoter to promote the formation of the cristobalite layer on the surface of the quartz glass crucible. In addition, a silica sol liquid containing a stabilizing reagent can also be used. The preferred metal salt is a metal organic acid salt or a metal carbonate.
The most preferable metal organic acid salts are carboxylates. As an acyloxy group to form the carboxylate, the materials indicated by the general formula CnH2n+1COO, where n is an integer of 3 to 7, are preferable. Concretely, the acyloxy group derived from n-butyric acid, xcex1-methyl butyric acid, iso-valeric acid, 2-ethyl butyric acid, 2,2-dimethyl butyric acid, 3,3-dimethyl butyric acid, 2,3-dimethyl butyric acid, 3-methyl pentanoic-acid, 4-methyl pentanoic acid, 2-ethyl pentanoic acid, 3-ethyl pentanoic acid, 2,2-dimethyl pentanoic acid, 3,3-dimethyl pentanoic acid, 2,3-dimethyl pentanoic acid, 2-ethyl hexanoic acid, or 3-ethyl hexanoic acid, etc., can be used suitably.
Moreover, when a xcex2-diketone, such as 2,4-pentanedione (=acetylacetone), 3-methyl-2,4-pentanedione, 3-isopropyl-2,4-pentanedione, or 2,2-dimethyl-3,5-hexanedione etc., is mixed with said carboxylate solution, the preservation stability of the liquid is improved.
It is preferable that these organic acid salts are dissolved in an organic solvent. As the suitable organic solvent, an ester and/or an alcohol, or a mixed solvent, where the carboxylic acid is further mixed with an ester and/or an alcohol, can be used. As the ester of the organic solvent, ethyl acetate, propyl acetate, n-butyl acetate, sec-butyl acetate, tert-butyl acetate, iso-butyl acetate, n-amyl acetate, sec-amyl acetate, tert-amyl acetate, and iso-amyl acetate, are preferable.
As an alcohol, methanol, ethanol, 1-propanol, 2-propanol, 1-butanol, 2-butanol, iso-butyl alcohol, 1-pentanol, 2-pentanol, 2-methyl-2-pentanol, and iso-amyl alcohol, etc., are suitable. In alcohols, an alkoxy alcohol, i.e., the alcohol containing an ether group, is included. For example, 2-methoxyethanol and 1-methoxy-2-propanol can be used.
As another example of a usable solvent, a ketone, such as acetone, methylethylketone, and methyisobutylketone, and a hydrocarbon, such as toluene, xylene, hexane, and cyclohexane, can be used. It is possible that a mixture of two and more kinds of these organic solvents is used. The solution containing a carboxylate of a metal is mixed with the solution containing the partial hydrolyzate of an alkoxysilane oligomer, to provide the predetermined composition, and the silica sol liquid is prepared. The silica sol liquid has high wettability to the surface of the crucible. The final hard and transparent silica coated layer, in which the metal oxide is dispersed in the silica matrix, can, be formed by baking to dehydrate said silica sol liquid after the organic components are removed completely.
The partial hydrolyzate of an alkoxysilane oligomer used in the present invention is obtained by a process in which the alkoxysilane oligomer is hydrolyzed under the controlled reaction conditions, and after the alcohol is removed, each generated OH group is bonded together to form the silica sol. As a suitable starting material, one or more silane compounds having at least one, preferably more than two, and more preferably more than three alkoxyl groups, can be used. Concretely, tetraethoxysilane (=ethylsilicate), tetrapropoxysilane, methyltriethoxysilane, dimethylmethoxysilane, phenyltriethoxysilane, chlorotrimethylsilane, various kinds of silane coupling reagents, such as vinyltriethoxysilane, xcex3-aminopropyltriethoxysilane, etc., also can be mentioned. Ethylsilicate, which is the cheapest and to be easily hydrolyzed, is preferred.
These alkoxysilanes are used as the hydrolyzate, which is partially hydrolyzed beforehand. The partial hydrolysis is carried out in the presence of an acid catalyst, i.e., an inorganic acid, such as hydrochloric acid, or an organic acid, such as p-toluene sulfonic acid, and water. Then, the reaction conditions, including, whether using the acid catalyst or not, the amount of acid catalyst, the amount of the water for hydrolysis in the reaction system, the reaction temperature and the reaction time, are adjusted to obtain the partial hydrolyzate of alkoxysilane oligomer, where the degree of polymerization is controlled. It is preferable that the partial hydrolysis is carried out with heating and stirring in an organic solvent containing water. In addition, it is preferable that the reaction temperature is 30 to 60xc2x0 C., more preferably 35 to 55xc2x0 C., and the reaction time is 2 to 5 hours.
Regarding the metal amount in said silica sol liquid, it is preferable that the metal amount in the coated layer is 1xc3x9710xe2x88x929 to 1xc3x9710xe2x88x926 mol/cm2, which is calculated as an oxide, when the coated silica sol liquid is baked to form said coated layer on the surface of the crucible. For example, it is preferable that the metal content of the silica sol liquid is 0.01 to 15 weight %, preferably 0.5 to 10 weight %, which is calculated as an oxide, and the silicon concentration is 0.5 to 30 weight %, which is calculated as SiO2. The solution containing the metal salt and the partial hydrolyzate solution of alkoxysilane oligomer, are mixed and prepared to provide the preferable metal content and silicon concentration. When the metal content is less than 0.01 weight %, which is calculated as an oxide, a repeat coating is necessary to form the desired devitrification layer thickness on the surface of the crucible. On the other hand, when the content is more than 15 weight %, it is difficult to make a stable mixed solution. Moreover, when the silicon concentration is less than 0.5 weight %, which is calculated as SiO2, the strength of the formed coated layer is not sufficient, and when the concentration is more than 30 weight %, it is difficult to make the mixed solution stable. It is preferable that the silicon concentration is 0.5 to 30 weight %, more preferably 1 to 20 weight %.
As the method for coating the silica sol liquid on the surface of the crucible a spray method and a dipping method, etc. can be used, and the coating method is not limited. In addition, the silica sol liquid on the surface of the crucible can be coated on a part or the whole of the inside surface or can be on a part or the whole of the outside surface; or can be on a part or the whole of both inside and outside surfaces. In order to increase the dislocation free ratio of the single crystal crystallizing the inside surface of the crucible during pulling up of the crystal, the silica sol liquid must be coated on at least the whole or a part of the inside surface of the crucible.
The silica sol liquid is coated on the surface of the crucible and is baked. It is suitable that the baking temperature is 350 to 1200xc2x0 C., preferably 600 to 1000xc2x0 C., and the baking time is 10 to 120 minutes. When the baking temperature is less than 350xc2x0 C., the strength of the coated layer is not sufficient, and when said temperature is more than 1200xc2x0 C., there is a possibility that the coated layer is devitrified. By this baking, the silica component of the silica sol liquid forms the hard silica matrix. Moreover, regarding the silica sol liquid containing the metal organic acid salt as the metal salt, the metal organic acid salt is decomposed into the metal oxide and the transparent coated layer in which the metal oxide is uniformly dispersed in the silica matrix, is formed on the surface of the crucible.
On the other hand, regarding the quartz glass crucible containing the metal carbonate, when the baking temperature is more than the decomposition temperature of the metal carbonate, the coated layer, where the metal carbonate is decomposed to the metal oxide to disperse in the silica matrix, is formed. On the other hand when the baking temperature is less than the decomposition temperature, the coated layer, where the metal carbonate is dispersed in the silica matrix a metal carbonate, is formed. In addition, when the baking temperature is near the decomposition temperature of the metal carbonate, the coated layer, where the metal oxide decomposed from a part of the metal carbonate and the non-decomposed metal carbonate are mixed, can be formed.